1. Field of the Invention
The present invention relates to an acoustic surface wave device. More specifically, the present invention relates to an acoustic surface wave device including a combination of a piezoelectric material substrate and interdigitated electrodes formed thereon, wherein an electrical signal is converted into an acoustic surface wave and vice versa.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a schematic diagram of a typical prior art acoustic surface wave device 100, which comprises a transducer 20 including a piezoelectric material substrate 10 of piezoelectric ceramic of such as PZT, a single crystal of such as LiNbO.sub.3, or a piezoelectric thin film of such as ZnO and an interdigital electrode 21 operatively coupled to the piezoelectric material substrate 10 and including a pair of groups of electrode fingers 22b, . . . 22b and 23b, . . . 23b formed on the surface of the piezoelectric material substrate 10 in an interdigitated manner and adapted to be in the same potential by means of a pair of common electrodes 22a and 23a. Typically, such acoustic surface wave device is structured such that the electrode fingers have been changed of the overlapping lengths of the adjacent electrode fingers that are overlapped with each other in the longitudinal direction of the electrode fingers in accordance with the predetermined weighting function for the purpose of achieving a desired pass characteristic. However, the velocity of the acoustic surface wave propagated along the piezoelectric material substrate is different between a region where the electrode fingers are more dominant and a region where the electrode fingers are less dominant, or a more weighted region and a less weighted region. Accordingly, in an acoustic surface wave device having the so called weighted interdigital electrodes, the velocity distribution of the acoustic surface wave becomes uneven in the plane orthogonal to the propagation direction of the acoustic surface wave, which gives rise to a phase difference at the output side of the acoustic surface wave device. Therefore, even the so called weighted acoustic surface wave device suffers from a disadvantage that a predetermined desired pass characteristic cannot be achieved.
Another prior art acoustic surface wave device of interest that eliminates the above described disadvantage to some extent is disclosed in U.S. Pat. No. 3,699,364, issued Oct. 17, 1972 to Henry M. Gerard and assigned to Hughes Aircraft Company, wherein dummy electrodes 231, . . . 231 and 221, . . . 221 are provided between the adjacent electrode fingers 22b, . . . 22b and 23b, . . . 23b in the non-weighted region in order to make uniform the distribution of the velocity of the acoustic surface wave. Provision of such dummy electrodes eliminates the difference in the velocity of the acoustic surface wave between the weighted region and the non-weighted region, resulting in a decreased phase difference at the output side of the acoustic surface wave device. However, provision of such dummy electrodes conversely increases an electrically and mechanically reflected wave of the acoustic surface wave, and as a result another problem is encountered wherein an undesired response, i.e. a ripple occuring in the pass band and the ripple of the group delay characteristic increase, as shown in FIG. 3 which shows a pass characteristic of a prior art acoustic surface wave device, the ordinate indicating the attenuation amount and the abscissa indicating the frequency of the acoustic surface wave.
In order to decrease an electrically reflected wave, it was proposed that dummy electrodes 211, . . . 221 and 231, . . . 231 are also coupled to the common electrodes 22a and 23a connecting the respective electrode fingers 22b, . . . 22b and 23b, . . . 23b to the same potential, as shown in FIG. 2, whereby the dummy electrodes are brought to the same potential as that of the adjacent electrode fingers. However, even in such an acoustic surface wave device wherein the dummy electrodes have been brought to the same potential as that of the adjacent electrode fingers, the above described ripple by virtue of a mechanically reflected wave is not improved at all. On the other hand, in fabricating such acoustic surface wave device, a piezoelectric material substrate of a larger electrical/mechanical coupling coefficient Keff with respect to the acoustic surface wave is employed for the purpose of improving the conversion efficiency. Since the above described mechanically reflected wave becomes larger in approximate proportion to a square of the electrical/mechanical coupling coefficient Keff, the above described mechanically reflected wave abruptly increases as the electrical/mechanical coupling coefficient Keff becomes larger. As a result, the ripple in the pass band and the ripple of the group delay characteristic (=-1/2.pi. D.phi./Df; where f is the frequency and .phi. is the phase) also abruptly increase with an increase in the electrical/mechanical coupling coefficient Keff, as shown by the dotted line in FIGS. 4 and 5, which will be described in more detail subsequently.